The nucleon structure program at COMPASS - ectstar.eu · The nucleon structure program at COMPASS . ... recently major progress on analysis ... the structure of the nucleon at LO

Anna Martin

Trieste University & INFN

on behalf of the COMPASS Collaboration

Lattice QCD and hadron physics

ECT*, 14-16 January, 2014

The

nucleon structure

program at COMPASS

fixed-target

experiment

at the CERN SPS

~ 230 physicists

data-taking since 2002

approved program

at least up to 2017

COmmon

Muon and

Proton

Apparatus for

Structure and

Spectroscopy

ECT* 2014 Anna Martin

several experiments using

COMPASS physics program

installed on the M2 CERN North Area beam line

unique possibility of using different high energy beams:

µ+, µ-, p, p+, p-, K+, K- → diversified physics program

muon beams

L and T polarised p and d targets

nucleon spin structure

- gluon and quark helicities

- transversity & TMD PDFs (SIDIS)

2002 – 2012

2015 – 2017

LH target

nucleon structure

- GPDs (DVCS and HEMP)

- FF and TMD effects (SIDIS)

hadron beams

LH and nuclear targets

pion polarisability

light meson and baryon

spectroscopy

T polarised p target

nucleon (spin) structure

- TMD PDFs (Drell-Yan)


a few results on

spectroscopy


rich program on hadron spectroscopy, search for exotic mesons

p, K, p beams - 190 GeV : large energy transfer spectrum t

spectrometer : flat acceptance, ECALs/ HCALs, RICH id.

charged & neutral channels

Pb 2004, Ni targets 2012

LH target with p recoil detector 2008-2009

huge statistics

recently major progress on analysis

physics with hadron beams


Diffractive Dissociation

• Isovector states: 𝜋𝐽, 𝑎𝐽

• Hybrids, exotics

• Scalar states as isobars

Central Production • Scalar states

• Glueballs

Photoproduction

• Polarizabilities: 𝜋

• Radiative couplings: 𝑎2, 𝜋2


pptarget ➝ pfast + X + precoil

➝ pfast + np/K/h + precoil

pptarget ➝ X + precoil

➝ np + precoil

for a review of the

new results see

B. Ketzer and other

COMPASS talks

at Hadron2013


Diffractive Dissociation

• Isovector states: 𝜋𝐽, 𝑎𝐽

• Hybrids, exotics

• Scalar states as isobars


pptarget ➝ X + precoil

➝ np + precoil

p- p+ p-

p- p0 p0

p- η η

p- p+ p+ p- p-


3p final state - kinematics


Isobar Model


1. PWA of angular distributions in 20 MeV mass bins and 11 t’ bins

2. Mass-dependent c2 fit to spin density matrix from step 1

total:

21 distributions x 100 mass bins x 11 t’ bins,

352 free parameters

2-D Partial Wave Analysis

new analysis method established: 2D-PWA in bins of 𝑚𝑋 and 𝑡′

• identify resonant and non-resonant contributions to partial waves

• precise extraction of resonance parameters

88 waves, well-known states as isobars

the high 2008/2009 statistics allows and demands for improved PWA methods


Preliminary Results of Fit to Spin-Density Matrix

• precise determination of resonance parameters

• mass and width ranges from systematic studies with different fit models

• final result and syst. error still under study

new axial vector meson observed: 𝑎1(1420) → 𝑓0(980)𝜋


COMPASS has collected large data samples with hadron beams

new analysis methods have been developed

more results to come in the near future



some results on

nucleon structure

160 GeV/c deuteron (6LiD) L & T polarisation 2002 − 2004

L polarisation 2006

160 GeV/c proton (NH3) L & T polarisation 2007

T polarisation 2010

longitudinally polarised μ+ beam

190 GeV/c proton (NH3) L polarisation 2011


longitudinal spin structure

of the nucleon


• direct measurement of Dg via Photon Gluon Fusion

COMPASS “flag-ship” measurement

longitudinal spin structure of the nucleon

LO

to select PGF process two methods are used:

• open-charm D meson production: charm quark pairs produced in PGF “clean” channel huge combinatorial background, low statistics

• high pT hadron pairs production: light quark pairs produced high statistics physical background; strongly model and MC dependent analysis


Dg results

LO

all results are consistent

and point toward

small gluon polarisation


• DIS, SIDIS on longitudinally polarised nucleons

Bjorken sum rule, Dq



DIS results

new: 2007 and 2011 p data

pA1


DIS results

new: 2007 and 2011 p data

pA1


SIDIS results


SIDIS results

large statistical uncertainties due to FFs

precise measurements of multiplicities are needed


from 2006 6LiD data

isoscalar target

hadron multiplicities

proton: from SIDIS

on LH2, 2016-2017


COMPASS has produced diversified, high precision results

in DIS and SIDIS down to x ~ 0.003

important contributions came and are coming from

HERMES, JLab and RHIC experiments


after more than 30 years the spin structure of the nucleon is understood

only qualitatively

a fast moving nucleon is a 3-dimensional complicated object

with nonnegligible orbital angular momentum contribution to its spin

unmeasured DVCS


transverse spin and momentum

structure of the nucleon

q


why SIDIS

taking into account the quark intrinsic transverse momentum kT , at leading order

8 TMD PDFs are needed for a full description of the nucleon structure

new objects,

giving the correlations between

spin of the nucleon,

spin of the quark,

transverse momentum of the

quark

today the only existing

informations come from

SIDIS, which give access to

all of them


A B

acc

he

tta e

t al., 2

00

6

cross-section

11T Dg

11T Hh

11 Hh

11 Hh

11T Df

11LHh

why SIDIS

18 structure functions

14 azimuthal modulations

all measured at COMPASS

on d, or on p and d


transversity and the TMD PDFs are new objects,

their properties have to be studied, the different effects disentangled

to be measured in a kinematical range as wide as possible

• large W

current jet and target fragmentation well separated

• small x

PDF parameterizations and their 1st moments

• large Q2 coverage

higher twist effects

why COMPASS

the high energy beam available at COMPASS ensures the

hardness of the process


high x, high Q2 :

luminosity limit

pμ = 160 GeV/c COMPASS

DIS event selection: Q2 > 1 (GeV/c)2

0.1 < y < 0.9

W > 5 GeV/c2

h± selection: pth > 0.1 GeV/c

z > 0.2 + low y low z

statistical error (raw asymmetry corrected for f)

6 months of data taking (June-November 2010)

complementary to

HERMES, JLab, JLab12


three distribution functions are necessary to describe

the structure of the nucleon at LO in the collinear case:

q(x) : number density or

unpolarised distribution

DTq(x) = q- q : transverse polarization or

transversity distribution

Dq(x) = q- q : longitudinal polarization or

helicity distribution

ALL OF EQUAL IMPORTANCE !

(x)fq

1

(x)gq

1

(x)hq

1

TRANSVERSITY

one of the main goals of COMPASS


DTq(x) = q- q : transverse polarization or

transversity distribution (x)hq

1

TRANSVERSITY

one of the main goals of COMPASS

it has different properties than helicity

tensor charge of the N

it survives after integration over the

intrinsic transverse momentum

it is chiral-odd, more difficult to measure than helicity (DIS)

SIDIS off transversely polarised nucleons:

q q

q

1

2

q

q 1

2

q

Dfe

Heq

“Collins FF”

Belle Babar CollA

q

1h


• clear non-zero effects first seen by HERMES on p in 2005

• ~ zero asymmetries measured by COMPASS on d

deuteron deuteron

2002-2004 NPB 765 (2007) 31, PLB 673 (2009) 127 2002-2004 NPB 765 (2007) 31, PLB 673 (2009) 127

deuteron

2002-2004 NPB 765 (2007) 31, PLB 673 (2009) 127

d

1

u

1 hh - unf

1

fav

1 HH -

still only

measurements on d

JLab : n (He3)

Collins asymmetry




Collins asymmetry

PLB 692 (2010) 240 PLB 717 (2012) 376 PLB 692 (2010) 240 PLB 717 (2012) 376 PLB 692 (2010) 240 PLB 717 (2012) 376

• COMPASS results on proton target (2007, 2010 data)




Collins asymmetry

PLB 692 (2010) 240 PLB 717 (2012) 376 PLB 692 (2010) 240 PLB 717 (2012) 376 PLB 692 (2010) 240 PLB 717 (2012) 376

• COMPASS results on proton target (2007, 2010 data)

good agreement with HERMES

in the x overlap region

same for pions

~ 3.8

~ 2.4


Transversity

transversity is different from zero and

can be measured in SIDIS thanks to the “Collins effect”

M. Anselmino et al., PRD87 (2013) 094019

simultaneous fit of

HERMES p, COMPASS p & d, and Belle data

very good χ2

A12 A0 2009

Bacchetta et al, 2012

model

calculations

more data

large and small x, p & d / n, PID

are needed


q q

q

1

2

q

q 1

2

q

Dfe

Heq

dihadron asymmetry

independent channel to access transversity in SIDIS on

transversely polarised nucleons

Collins

“Collins FF”

Belle Babar q

1h

q

2h

q

q

1

2

q

q q

2

q

Dfe

He

“Interference / Di-hadron FF”

Belle Babar

“spin independent di-hadron FF”

being measured at COMPASS

CollA

RSA

q

1h

dihadron


dihadron multiplicities in SIDIS dihadron multiplicities in SIDIS hadron pair multiplicities

in Minv , z=z1+z2 , Q2 bins

SPIN2012


all d data: asymmetries compatible with zero PLB 713 (2012) 10

dihadron asymmetry

high statistics over a wide x range

clear signal

all p data PLB 713 (2012) 10 & Transversity 2011

proton

preliminary h+ h-


dihadron asymmetry and Collins asymmetries

“intriguing” results

• Collins asymmetry for h+ and for h-

“mirror symmetry” preliminary

• dihadron asymmetry vs

Collins asymmetries

only somewhat larger

further investigation:

correlations between the relevant

azimuthal angles and the

corresponding asymmetries

information on the nature of the

fragmentation of transv polarised q

Collins vs 2h interference mechanism

F. Bradamante , Como2013, D-SPIN2013

conclusion:

hints for a common physical origin for the Collins mechanism

and the polarised dihadron fragmentation function

published


Sivers PDF

the most famous of the TMD PDFs

correlation between the transverse spin of the nucleon

and the transverse momentum of the quark

sensitive to orbital angular momentum

T-odd

SIDIS on transversely polarised nucleons

Sivers asymmetry

q q

q

1

2

q

q q

2

q

Dfe

De q

1

TfSivA


Sivers asymmetry

• strong signal seen by HERMES in p+ production on p in 2005

• no signal seen by COMPASS on d u ~ - d

2007, 2010 PLB 692 (2010) 240 PLB 717 (2012) 383

• COMPASS results on proton:

clear signal for h+

down to low x, in the previously unmeasured region

in the overlap x range, agreement with HERMES, but

clear indication that the strength decreases:

Q2 evolution


Sivers PDF

• from SIDIS, clear evidence that it is different from zero

• all the existing data can be well described in the present

theoretical framework

being T-odd, it should be process-dependent

it is expected to change sign if measured in Drell-Yan

)()( 11 DYfSIDISf TT -

future measurements of Drell-Yan at COMPASS

SIDIS


Boer-Mulders PDF

measured by COMPASS using combined data taken with

oppositely polarised 6LiD target

twist3 + …

Boer-Mulders PDF x Collins FF

+ Cahn effect (twist 4, 1/Q2)

Boer-Mulders PDF

Cahn effect, kT

• correlation between the transverse spin and

the transverse momentum of the quarks

in unpolarised nucleons

• T-odd

• can be accessed in SIDIS on unpolarised nucleons


cos 2

• large signals

at small x

cos

• large signals

over all

the x range

SPIN2010

different for h+ and h-

strong unexpected dependence on x, z, PTh

unpolarised deuteron - azimuthal asymmetries

sys ≈ 2 ∙ stat


unpolarised deuteron - azimuthal asymmetries

sys ≈

2 ∙ stat

x

pT

preliminary

h-

h+

UU

hA 2cos


unpolarised deuteron

hadron multiplicities vs pT2

0.0052

1.11

0.0148

1.92

0.0148

1.22

0.0099

1.76

0.0099

1.22

0.0070

1.48

0.0070

1.14

0.0213

2.94

0.0213

1.92

0.0213

1.23

0.0150

2.90

0.0216

4.07

0.0316

1.34

0.0318

1.92

0.0319

2.95

0.0323

4.47

0.0447

1.93

0.0533

2.95

0.0536

4.57

0.0550

7.36

0.0932

7.57

0.0921

4.62

Q2

x

‹ x ›

‹ Q2 ›

z bins:

0.20÷0.25

0.30÷0.35

0.40÷0.50

Eur.Phys.J.

C73 (2013) 2531


nucleon structure

future COMPASS contribution

• the analysis of the SIDIS is not over

• further investigation of single hadron and dihadron asymmetries

• multidimensional analysis of transverse spin asymmetries

• more d and p results on longitudinal spin azimuthal asymmetries

• more on azimuthal asymmetries and multiplicities from unpolarised d data (PID)

• ….

new results coming soon

• future measurements


GPDs from DVCS and HEMP

2016 - 2017


GPD program at COMPASS

• transverse target spin asymmetry for

exclusive ρ0 production on d and p

• DVCS and Hard Exclusive Meson Production

with LH2 target and +, - 160 GeV beams

to constrain GPD H

to study the transverse proton structure

data taking 2016-2017

2002-2004, 2007, 2010 data

test runs

2009,2012

NPB 865 (2012) 1, CERN-PH-EP/2013-191

ECT* 2014 Anna Martin B

kinematic domain (Q2, xB) for DVCS

fixed target

collider

explore the

intermediate xBj region

uncovered region between

ZEUS+H1 & HERMES + JLab

before new colliders may be

available

COMPASS unique facility

CERN muon beam

• 100 - 190 GeV

• μ+ and μ- available

• 80% Polarisation

• 4.6 108 +

Lumi= 1032 cm-2 s-1

with 2.5m LH2 target


experimental apparatus

SM1

SM2

ECAL1

ECAL2

ECAL0

CAMERA

new equipements

• 2.5m LH2 target

• 4m ToF Barrel CAMERA

• ECAL0

• tracking detectors


CAMERA recoil proton detector

surrounding the 2.5m long

LH2 target

ECAL0

ECAL2

ECAL1

DVCS test run 2012


SIDIS in parallel to DVCS

2016 - 2017

q


SIDIS

on the 2.5 m long LH2 target with the 160 GeV/c muon beam

taking advantage of the spectrometer consolidation and

upgrades which are ongoing trackers, RICH

• hadron multiplicities vs z FFs s quark PDFs

• dihadron multiplicities transversity

• TMDs

hadron multiplicities vs pt2 kT

azimuthal asymmetries BM PDF

all relevant measurements


polarised Drell – Yan 2015

test run in 2009


Drell - Yan

complementary to SIDIS

𝝅−𝒑 → 𝝁+𝝁−𝑿

q

cross-sections:

SIDIS: convolution of a TMD PDFs with FFs

DY: convolution of 2 TMD PDFs


Drell – Yan at COMPASS

𝝅−𝒑↑ → 𝝁+𝝁−𝑿

aim: test for the first time the expected

change of sign of the Sivers function

)()( 11 DYfSIDISf TT -

never measured clearly positive

transversely polarised p target

SIDIS


polarised Drell - Yan

acceptance

statistical errors

24 9 /M GeV c

𝝅−𝒑↑ → 𝝁+𝝁−𝑿


polarised Drell - Yan 𝝅−𝒑↑ → 𝝁+𝝁−𝑿

overlap of the x-Q2 region covered in the two processes overlap

consistent extraction of TMD PDFs

Q2 vs x phase space at COMPASS

190 GeV π


COMPASS

great impact results

on nucleon structure and spectroscopy

have already been obtained and

new ones are expected in the next few years

more can be done later on - from 2018

first ideas

submitted to European Strategy Preparatory Group, 2012

Thank you

Documents

The nucleon structure program at COMPASS - ectstar.eu · The nucleon structure program at COMPASS . ... recently major progress on analysis ... the structure of the nucleon at LO